Thermocouples are analog temperature sensors that utilize the thermoelectric properties of two dissimilar materials, typically metals, to generate an EMF in proportion to a temperature gradient across a material inhomogeneity. Common thermocouples used in temperature measurement comprise two metal wires of different thermoelectric properties called thermoelements connected at one end to form a “hot junction” also known as a “measuring junction”. The other ends of the wires are connected to instrumentation such as a voltmeter to measure the EMF produced by the thermocouple. The wires are connected to the instrumentation at a known reference temperature to form a “reference junction” or a “cold junction”. For the most precise measurements it is desirable that the only material inhomogeneity in the measurement circuit occurs at the measurement junction where the two dissimilar materials are joined.
Because it is undesirable to have any other EMF sources between the measurement junction and the reference junction, it is important that there is a minimal temperature gradient across the thermoelectric material and the electrical instrumentation leads. Thermocouple materials are specialized alloys while electrical instruments typically utilize common metals such as copper, nickel, gold, beryllium copper, aluminum, and a variety of plating materials. This material inhomogeneity at the reference termination can lead to significant errors unless care is taken to minimize temperature gradients in this region or to accurately characterize the gradients that exist.
In addition, thermocouple assemblies must use expensive hardware in the connection schemes at the termination end. Rather than using common contact materials such as copper, nickel, gold, and others that are readily available, thermocouple connectors are made from more expensive thermocouple materials to minimize any inhomogeneity in the connector. A connector using thermocouple materials for the pins and sockets is referred to as a compensated connector. A compensated connector is designated to work only with a specific thermocouple type thus limiting the utility of the electronic instrumentation to only one type of sensor.
In thermocouple temperature measurement it is important to accurately establish the temperature of the reference junction in order to determine the temperature of the measured junction. In industrial temperature measurement, a temperature sensor such as an RTD, thermistor, diode, transistor, or an IC chip type sensor measures the cold junction. In almost all instances there will exist a temperature gradient between the cold junction sensor location and the location of the thermocouple leadwire termination. This temperature gradient is usually traversed by non-thermocouple wires or circuit board traces that generate little or no EMF. The result of this is an error in the temperature measurement that is approximately equal to the size of the temperature gradient.
A solution to the problem of a temperature gradient existing between the cold junction compensation (CJC) sensor and the thermocouple termination would be to use compensated materials for the lead connection and terminals. For example, a copper/constantan thermocouple (Type T) could use circuit board and terminals made from copper for the positive leg and constantan for the negative leg. This solution, however, has several negative aspects. The first and most obvious negative impact is the system is now only suitable for type T thermocouples because other types of thermocouples would experience an error from the different EMFs generated by the material differences. A second problem is a more practical problem of material availability. While copper is commonly available for terminals and printed circuit board traces, the other material, constantan, is not. Electrical hardware made from constantan or other common thermocouple alloys would be costly and would not be readily available.
Accordingly, there is a need for compensating the cold junction of a thermocouple system using low cost materials for system components accommodating various thermocouple types.